Communication terminal that performs network packet communication using sip servers, control method for the communication terminal, and storage medium

ABSTRACT

A communication terminal that determines the types of SIP servers, and stores SIP server information in association with the determined types so that SIP communication can be normally performed. The communication terminal carries out network packet communication using the SIP servers. SIP server information on the SIP servers is acquired, and types of the SIP servers are determined based on the acquired SIP server information. The SIP server information is stored in association with the determined types of the SIP servers.

This application is a U.S. National Phase Application of PCTInternational Application PCT/JP2011/064616 filed on Jun. 21, 2011 whichis based on and claims priority from JP2010-153815 filed on Jul. 6, 2010the contents of which is incorporated herein in its entirety byreference.

TECHNICAL FIELD

The present invention relates to a communication terminal that performsnetwork packet communication using SIP servers, a control method for thecommunication terminal, and a storage medium storing a program forexecuting the method.

BACKGROUND ART

In recent years, wide-area Ethernet (registered trademark) and IP-VPN(internet protocol-virtual private network) have been becomingincreasingly advanced and widespread, and environments where WAN(wide-area network) is realized by an IP network in place of aconventional dedicated line or ATM (asynchronous transfer mode) havebeen increasing. Accordingly, voice service, which has conventionallybeen realized by PSTN (public switched telephone network) using PBX(private branch exchange), has come to be realized by VoIP using VoIP(voice over IP) gateway or a VoIP adapter. In particular, SIP (sessioninitiation protocol) based on text descriptions with a high affinity forHTTP (Hypertext Transfer Protocol) has becoming more common as asignaling protocol for realizing voice service using VoIP as compared toH.323 whose data structure is complicated.

In SIP environments, SIP servers are commonly placed with an SIPterminal, and SIP servers offer functions such as registration and nameresolution of an SIP server address, and call routing. SIP servers maybe placed in a public network of an ISP (Internet service provider), orplaced in a private network of a company.

SIP servers are broadly classified into three types according tofunctions. The first SIP server is an SIP registration server thatcarries out registration and name resolution of an SIP server address.The second SIP server is an SIP proxy server that acts as an alternativeto an SIP terminal to carry out SIP communication with an opposite SIPterminal and a VoIP gateway. The third SIP server is an SIP redirectserver that sends back an opposite destination that carries out callrouting suitable for an SIP terminal. There is known a network system inwhich these SIP servers are placed separately for individual functions(see, for example, PTL (Patent Literature) 1).

When different SIP servers are placed for different functions, a normalresponse cannot be obtained from an SIP server unless an appropriate SIPmessage (an SIP message suitable for the type of the SIP server) istransmitted from an SIP terminal to an appropriate server. For example,an error is caused by transmission of a message requesting the start ofSIP communication to an SIP registration server, and also, an error iscaused by transmission of a message registering an SIP server address toan SIP proxy server. For this reason, SIP communication may not becarried out unless appropriate SIP server settings are configured at anSIP terminal.

If settings including the types of SIP servers are manually configured,the above problem can be avoided because appropriate SIP servers can bemanually set one by one.

CITATION LIST Patent Literature

-   {PTL 1} Japanese Laid-Open Patent Publication (Kokai) No.    2008-033558

SUMMARY OF INVENTION Technical Problem

As described above, when settings including types of SIP servers aremanually configured, a lot of time and effort are required for a user.On the other hand, it is known that SIP server information isautomatically acquired using DHCP (dynamic host configuration protocol)or DHCPv6 as defined by RFC 3361 or the like. Further, it is known thatSIP server information is automatically acquired using multicastsearches or broadcast searches.

However, when SIP server information is automatically acquired, noinformation is added to the types of SIP servers. For this reason, thetypes of retrieved SIP servers cannot be determined, and erroneous SIPmessages (SIP messages unsuitable for the types of the SIP servers) maybe transmitted to the SIP servers.

The present invention provides a communication terminal that determinesthe types of SIP servers, and stores SIP server information inassociation with the determined types so that SIP communication can benormally performed, a control method for the communication terminal, anda storage medium storing a program for implementing the method.

Solution to Problem

Accordingly, a first aspect of the present invention provides acommunication terminal that carries out network packet communicationusing SIP servers, comprising an acquisition unit that acquires SIPserver information on the SIP servers, a determination unit thatdetermines types of the SIP servers based on the SIP server informationacquired by the acquisition unit, and a storage unit that stores the SIPserver information acquired by the acquisition unit in association withthe types of the SIP servers determined by the determination unit.

Accordingly, a second aspect of the present invention provides a controlmethod for a communication terminal that carries out network packetcommunication using SIP servers, comprising an acquisition step ofacquiring SIP server information on the SIP server, a determination stepof determining types of the SIP servers based on the SIP serverinformation acquired in the acquisition step, and a storage step ofstoring, in a storage unit, the SIP server information acquired in theacquisition step in association with the types of the SIP serversdetermined in the determination step.

Accordingly, a third aspect of the present invention provides Anon-transitory computer-readable storage medium storing a program forcausing a computer to implement a control method for a communicationterminal as described above.

Advantageous Effects of Invention

According to the present invention, because types of SIP servers aredetermined, and SIP server information is stored in association with thedetermined types, SIP communication can be normally performed.

The features and advantages of the invention will become more apparentfrom the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically showing an arrangement of an SIPcommunication network system according to an embodiment of the presentinvention;

FIG. 2 is a diagram schematically showing an exemplary UI for acquiringand setting SIP sever information at an SIP terminal;

FIG. 3 is a block diagram schematically showing a hardware arrangementof the SIP terminal;

FIG. 4 is a block diagram schematically showing a software arrangementof the SIP terminal;

FIG. 5 is a flowchart of an SIP information acquisition process carriedout by an SIP server information automatic acquisition unit;

FIG. 6 is a diagram showing an exemplary DHCP reply packet in a casewhere the SIP server information automatic acquisition unit hassuccessfully acquired SIP server information using DHCPv4;

FIG. 7 is a flowchart of an SIP information determination processaccording to a first embodiment, which is carried out by an SIP serverinformation determination unit;

FIG. 8 is a diagram showing an exemplary body part of a transmittedpacket of an OPTIONS method, which is transmitted by an SIP serverinformation determination unit;

FIG. 9 is a diagram showing an exemplary body part of a response packetto the transmitted packet shown in FIG. 8;

FIG. 10 is a flowchart of an SIP information determination processaccording to a second embodiment, which is carried out by the SIP serverinformation determination unit; and

FIG. 11 is a diagram showing exemplary SIP response codes used in theSIP information determination process.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings showing an embodiment thereof.

FIG. 1 is a block diagram schematically showing an SIP communicationnetwork system according to an embodiment of the present invention. AnSIP terminal 100, which is a communication terminal connected to a firstLAN (local area network) 109 constituting the SIP communication networksystem, has an SIP call control function. The DHCP server 101 has atleast a function of supporting at least an option 120, and distributingSIP server information. It should be noted that the “options 120” is astandard relating to SIP servers managed by the IRNA (internet assignednumbers authority).

An SIP registration server 102 accepts a process for registration of anSIP server address or the like from the SIP terminal 100. An SIP proxyserver 103 acting as a proxy for the SIP terminal 100 to perform SIPcall control with SIP terminal at the other end (not shown). An SIPredirect server 104 receives requests from the SIP terminal 100, andprovides notification about an address of the opposite SIP terminal.

A VoIP gateway 105 converts IP packets into analog voice data. A PBX106, which is a private branch exchange, is connected to an analogtelephone 108 and others, and also connected to a public line via a PSTN111. The LAN 109 is connected to the Internet 110 via a router 107, andconnected to other LANs such as a second LAN 112 and a third LAN 113 viathe Internet 110. The IP packets can be communicated between the LAN 109and the Internet 110 via the router 107.

FIG. 2 is a diagram schematically showing an exemplary UI (userinterface) for acquiring and setting SIP sever information at the SIPterminal 100. In “SIP server acquisition method” in FIG. 2, a user canselect any one of “manual acquisition” that SIP server information ismanually set, “acquisition from DHCP” that SIP server information isacquired from the DHCP server 101, and “multicast search” that amulticast search is performed.

As SIP servers, three types of servers consisting of an SIP registrationserver, an SIP proxy server, and an SIP redirect server can be set. Foreach server, a primary and a secondary can be set, and each of them canbe set in the form of an IPv4 address, an IPv6 address, a host name, anSIP address, or the like.

When “manual” is selected in “SIP server acquisition method”, the usermanually types values in address fields for each server, and the inputcan be done via an operation panel 309 (see FIG. 3), to be describedlater. On the other hand, when “acquisition from DHCP” or “multicastsearch” is selected, automatic acquisition of SIP server information isperformed at the time of startup, and acquired SIP server information isstored.

FIG. 3 is a block diagram schematically showing a hardware arrangementof the SIP terminal 100. A CPU 301 executes software programs for theSIP communication network system, and controls the entire SIP terminal100. A RAM 302 is used for temporary data storage when the CPU 301controls the apparatus. A ROM 303 stores a boot program, fixedparameters, and so on for the SIP terminal 100. An HDD 304 storesvarious data and programs. It should be noted that the HDD 304 is merelyan example of a storage device, and any other storage device (forexample, a flash memory or a DVD-RAM) may be used instead.

A timer 308 manages elapsed time in timer processing. An NVRAM 305stores various setting values for the SIP communication network systemsuch as information relating to SIP. A panel control unit 306 controlsthe operation panel 309 to input instruction from a user to displayvarious information. It should be noted that the screen in FIG. 2 foracquiring and setting SIP server information is displayed on theoperation panel 309, and the user may confirm set SIP server informationvia the display on the operation panel 309. A network I/F unit 307carries out network packet communication via the LAN 109.

FIG. 4 is a block diagram schematically showing a software arrangementof the SIP terminal 100. Functions of components shown in FIG. 4 arerealized by the CPU 301 performing computations based on predeterminedprograms stored in the ROM 303 and so on or data stored in the HDD 304,the NVRAM 305, and so on, and operating various hardware constitutingthe SIP terminal 100.

An SIP control unit 401 provides overall control relating to SIPcommunication such as creation and interpretation of SIP messages. AnSIP server information automatic acquisition unit 402 automaticallyacquires SIP server information via the LAN 109 using DHCPv4 and DHCPv6,or acquires SIP server information by transmitting a multicast packet ora broadcast packet via the LAN 109. An SIP server informationdetermination unit 403 receives SIP server information from the SIPserver information automatic acquisition unit 402 to determine the typesof SIP servers, and stores appropriate SIP server information in an SIPserver information storage and setting unit 408 in association with thetypes of the SIP servers, and notifies the SIP control unit 401 of thesame.

An SIP information registration unit 404 carries out a process toregister information on the SIP terminal 100 in the SIP registrationserver 102. An SIP library unit 405 offers a library for an SIPapplication unit 406 to realize SIP communications. The SIP applicationunit 406 is a group of applications realized on SIP, and examples of theapplications include SIP-FAX, IP phone, and SIP printing.

An UI control unit 407 stores SIP server information set via the UI bythe user in the SIP server information storage and setting unit 408, andnotifies the SIP control unit 401 of the same. The SIP serverinformation storage and setting unit 408 stores SIP server informationacquired by the SIP server information determination unit 403 oracquired via the UI, and sets the SIP server information in the SIPterminal 100. A protocol stack 409 controls IP and TCP/UDP in the SIPcommunication network system, and acts as a NIC control unit thatcontrols a NIC (network interface card) that carries out network packetcommunication via the LAN 109.

FIG. 5 is a flowchart of an SIP server information acquisition processcarried out by the SIP server information automatic acquisition unit402. When the SIP terminal 100 is started, the SIP server informationautomatic acquisition unit 402 determines how to acquire SIP serverinformation from SIP server information acquisition settings (stepS501). It should be noted that the SIP server information acquisitionsettings are configured in advance via the operation panel 309 by theuser, and stored in the NVRAM 305 (or the HDD 304) in accordance withFIG. 2.

When in the step S501, it is determined that SIP server information isto be manually acquired, the SIP server information automaticacquisition unit 402 notifies the SIP control unit 401 to that effect(step S502), and then terminates the process. It should be noted thatthe SIP control unit 401 displays, on the operation panel 309, thescreen in FIG. 2 for acquiring and setting SIP server information, andprompts the user to set SIP server information.

When in the step S501, it is determined that SIP server information isto be acquired from DHCP, the SIP server information automaticacquisition unit 402 transmits a DHCP request including the option 120to the DHCP server 101 (step S503). Then, the SIP server informationautomatic acquisition unit 402 interprets a DHCP response which isresponse data to the DHCP request, and determines whether or not SIPserver information has been successfully acquired (step S505).

When, for example, there is no response from the DHCP server 101, orerror information is included in the DHCP response, it is determinedthat the SIP server information has not been successfully acquired (“NO”in the step S505). In this case, the SIP server information automaticacquisition unit 402 notifies the SIP control unit 401 and the UIcontrol unit 407 of unsuccessful acquisition (step S507). Upon receivingthe notification of unsuccessful acquisition, the UI control unit 407displays a message saying that “SIP server information has not beensuccessfully acquired”, which is not shown, on the operation panel 309to notify the user to that effect. Then, the process is terminated.

When SIP server information has been successfully acquired (“YES” in thestep S505), the SIP server information automatic acquisition unit 402notifies the SIP information determination unit 403 of the SIP serverinformation (step S506), and then terminates the process. FIG. 6 showsan exemplary DHCP reply packet in a case where the SIP serverinformation has been successfully acquired using DHCPv4. According tothe DHCP reply packet shown in the figure, three lists (IP addresses)have been acquired as SIP servers, but the types of the SIP servers arenot defined. A description will be given later of how the types of theSIP servers are defined by the SIP server information determination unit403.

When in the step S501, it is determined that SIP server information isto be acquired using a multicast search, the SIP server informationautomatic acquisition unit 402 transmits a multicast packet forsearching an SIP server (step S504). It is then determined whether ornot SIP server information has been successfully acquired (step S505).When there is no response to the multicast packet or when SIP serverinformation has not been successfully acquired (“NO” in the step S505),the same error processing as in the case of acquisition from DHCP iscarried out (step S507). When SIP server information has beensuccessfully acquired (“YES” in the step S505), the same successprocessing as in the case of acquisition from DHCP is carried out (stepS506).

The above described process may be carried out for not only the option120 but any information as long as the information relates to SIPservers. DHCP includes DHCPv4 and DHCPv6, and the present embodiment maybe applied to both protocols. Multicast packets include IPv4 and IPv6multicast packets, and the present embodiment may be applied to bothprotocols.

FIG. 7 is a flowchart of an SIP information determination processaccording to a first embodiment, which is carried out by the SIP serverinformation determination unit 403. The SIP server informationdetermination unit 403 determines whether or not it has been notified ofSIP server information by the SIP server information automaticacquisition unit 402 (step S601), and waits until it is notified of SIPserver information (“NO” in the step S601). When notified of SIP serverinformation (“YES” in the step S601), the SIP server informationdetermination unit 403 determines whether or not the types of SIPservers can be determined from the SIP server information (step S602).

When the types of SIP servers can be determined from the SIP serverinformation (“YES” in the step S602), the SIP server informationdetermination unit 403 stores information on the types of SIP serversand address information in the SIP server information storage andsetting unit 408, and notifies the SIP control unit 401 of the same(step S611). As a result, the SIP server information is set in the SIPterminal 100, and appropriate SIP server information is set in the UI inFIG. 2, so that the user can confirm the set SIP server information viathe UI.

When the types of the SIP servers cannot be determined (“NO” in the stepS602), and an SIP server information list as shown in FIG. 6 is posted(step S603), the SIP server information determination unit 403 cannotdetermine the types of the SIP servers from contents of the SIP serverinformation list. Thus, to determine the types of the SIP servers, theSIP server information determination unit 403 transmits an OPTIONSmethod, which is one of SIP methods, to each of the SIP servers on thelist (step S604).

FIG. 8 is a diagram showing an exemplary body part of a transmittedpacket of the OPTIONS method transmitted by the SIP server informationdetermination unit 403. OPTIONS is stored in a Cseq field, and an SIPserver address or a host name posted from the SIP server informationautomatic acquisition unit 402 is stored in a TO field. Formats of otherfields comply with RFC standards for SIP.

FIG. 9 is a diagram showing an exemplary body part of a response packetto the transmitted packet shown in FIG. 8. A list of SIP methodssupported by the concerned SIP server is stored in an Allow field. Inthe example shown in FIG. 9, SIP methods Invite, Ack, Bye, OPTIONS,Cancel, Refer, and Register are supported. The SIP server informationdetermination unit 403 determines the types of the SIP servers fromthese fields.

After the step S604, it is determined whether or not the Register methodis included in the Allow field (step S605). When the Register method isincluded in the Allow field (“YES” in the step S605), the concerned SIPserver acts as an SIP registration server, and it is thus determinedthat the concerned SIP server is an SIP registration server (step S607).Then, the process proceeds to perform processing on the next SIP serverlist, and when there is no SIP server list (step S610), the processproceeds to step S611.

When the Register method is not included in the Allow field (“NO” in thestep S605), it is determined whether or not the Refer method is includedin the Allow field (step S606). When the Refer method is included in theAllow field (“YES” in the step S606), the SIP server acts as an SIPproxy server, and it is thus determined that the concerned SIP server isan SIP proxy server (step S608). When the Refer method is not includedin the Allow field (“NO” in the step S606), it is determined that theconcerned SIP server is an SIP redirect server (step S609). After thesteps S608 and S609, the process proceeds to perform processing on thenext SIP server list, and when there is no SIP server list (step S610),the process proceeds to the step S611.

After the types of all the SIP servers posted in the step S603 have beendetermined, the SIP server information determination unit 403 storesinformation on SIP server addresses and types in the SIP serverinformation storage and setting unit 408, and notifies the SIP controlunit 401 of the same (step S611). As a result, the SIP serverinformation is set in the SIP terminal 100, and the UI in FIG. 2 isupdated with appropriate SIP server information, so that the user canconfirm the SIP server information set for each SIP server type via theUI.

It should be noted that when an upper limit to the number of SIP serversset in the SIP terminal 100 has been reached although types of all theSIP servers posted in the step S603 have not yet been determined, theSIP server type determination process may be brought to an end, and theprocess may proceed to the step S611.

According to the above described process, when SIP server information isto be automatically set, the SIP control unit 401 receives notificationafter the types of appropriate SIP servers have been determined.Moreover, as described earlier, when the user sets SIP serverinformation via the UI control unit 407 in a case where SIP serverinformation is to be manually set, the UI control unit 407 notifies theSIP control unit 401 of the set SIP server information.

In either the case where SIP server information is to be manually set orthe case where SIP server information is to be automatically set, theSIP control unit 401 determines that SIP communications are possibleafter completion of notification to that effect, and starts the SIPinformation registration unit 404 and the SIP application unit 406, orprovides them with notification that preparations for startup have beencompleted. As a result, the process for registering SIP server addressestransmitted by the SIP information registration unit 404 is successfullycompleted because the SIP server addresses are transmitted to theappropriate SIP registration server 102 in FIG. 1. Moreover, when theSIP application unit 406 requests SIP proxy communication, this requestis transmitted to the SIP proxy server 103, and when the SIP applicationunit 406 requests redirect communication, this request is transmitted tothe SIP redirect server 104. In this way, communication with appropriateSIP servers can be realized.

The SIP server type determination process described above has only to becarried out only once when the SIP terminal 100 is started, and does nothave to be carried out each time an SIP message is transmitted, and thusthere is no degradation in the performance of SIP transmission.

Although in the above description, the type of an SIP servers isdetermined when SIP server information is acquired, the type of an SIPserver may be determined when an SIP message is transmitted. In thiscase, however, it may take time to transmit an SIP message, and this maycause degradation in the performance of transmission. On the other hand,if the type of an SIP server is determined when SIP server informationis acquired as described above, degradation in the performance oftransmission can be prevented.

According to a second embodiment, an SIP server information automaticacquisition method is the same as the method described above withreference to FIG. 5 showing the first embodiment, and therefore,description thereof is omitted here. FIG. 10 is a flowchart of an SIPinformation determination process according to the second embodiment,which is carried out by the SIP server information determination unit403.

Although in the first embodiment, the type of an SIP server isdetermined using a response to the OPTIONS method, it is conceivablethat there may be an invalid response or no response to the OPTIONSmethod, and thus the type of an SIP server may not be appropriatelydetermined. For example, it is conceivable that there may be no responseto OPTIONS when Register is included in the Allow field because the SIPserver is an SIP registration server, or the SIP server may not respondto the OPTIONS method itself. The second embodiment relates to adetermination method that can avoid the above described problems andappropriately determine the type of an SIP server.

In the second embodiment, the OPTIONS method, which is used in the firstembodiment, is not used for determining the types of an SIP server.Processes in steps S701 to S703, S711, and S712 in FIG. 10 are the sameas the steps S601 to S603, S610, and S611 in FIG. 10, and therefore,detailed description thereof is omitted here. Thus, detailed descriptionwill be now given of processes in steps S704 to S710.

After the process in the step S703, the SIP server informationdetermination unit 403 transmits a Register method to a target SIPserver (step S704). When the method in the step S704 is successful, itis determined that the SIP server is an SIP registration server (stepS706). When the method in the step S704 is unsuccessful, an Invitemethod that stores an appropriate destination SIP server address istransmitted to the target SIP server (step S705).

When, as a result of the step S705, a response code of a response packetis in the 300s (3XX), it is determined that the SIP server is an SIPredirect server (step S707). On the other hand, when, as a result of thestep S705, a response code of a response packet is in the 200s (2XX), Itis determined that the concerned SIP server is an SIP proxy server (stepS708).

However, when, as a result of the step S705, a response code of aresponse packet is in the 600s (6XX), the type of the SIP server cannotbe determined. Therefore, the determination is terminated, and theprocess proceeds to perform processing on the next server list. When anyresponse code other than the above-mentioned ones is received, the SIPserver information determination unit 403 stands by for a predeterminedperiod of time, and performs a retry (step S710).

FIG. 11 is a diagram showing exemplary SIP response codes, which aredefined by RFC. After completion of the processes in the steps S706 toS709, the process proceeds to the step S711 and the subsequent steps asis the case with the first embodiment.

Although the embodiments of the present invention has been described byway of illustration, the present invention is not limited to theembodiments described above. For example, it is preferred that the useris allowed to selectively determine whether to practice the firstembodiment or the second embodiment via a UI. Also, the first embodimentor the second embodiment may be practiced in combination suitably for auser network environment. In this case, weights may be assignedregarding which embodiment should be practiced first, which embodimentis given a higher priority in terms of practice result, and so on, andthe user may be allowed to configure such settings.

Although in the above description of the embodiments, an SIPregistration server, an SIP proxy server, and an SIP redirect server aregiven as examples of SIP server types, the present invention may beapplied to any types of SIP servers such as an SIP location server.Regarding the type of an SIP server as well, the user may be allowed toselect any desired type from a plurality of SIP servers, or newlyregister any desired type.

Automatic acquisition of SIP server information may be carried out onlyonce when the SIP terminal 100 is started as in the first and secondembodiments, or may be carried out at a time set by the user using thetimer 308, or carried out at regular time intervals set in the timer308. Further, any desired trigger such as the timing in which an SIPserver address is changed may be set in advance by the user, and SIPserver information may be automatically acquired in the timing in whichthe trigger occurs.

Other Embodiments

Aspects of the present invention can also be realized by a computer of asystem or apparatus (or devices such as a CPU or MPU) that reads out andexecutes a program recorded on a memory device to perform the functionsof the above-described embodiment(s), and by a method, the steps ofwhich are performed by a computer of a system or apparatus by, forexample, reading out and executing a program recorded on a memory deviceto perform the functions of the above-described embodiment(s). For thispurpose, the program is provided to the computer for example via anetwork or from a recording medium of various types serving as thememory device (e.g., computer-readable medium).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

REFERENCE SIGNS LIST

-   100 SIP terminal-   102 SIP registration server-   103 SIP proxy server-   104 SIP redirect server-   401 SIP control unit-   402 SIP server information automatic acquisition unit-   403 SIP server information determination unit-   109 LAN

The invention claimed is:
 1. A communication terminal that carries outnetwork communication using a SIP server, comprising: a reception unitthat receives, via a network, a SIP server address of the SIP server; atransmission unit that transmits a SIP method for inquiring about a typeof a supported SIP method which the SIP server supports using the SIPserver address received by said reception unit; a determination unitthat determines a type of the SIP server based on the type of thesupported SIP method indicated in response data to the SIP methodtransmitted by the transmission unit, the type of the SIP serverincluding at least one of a SIP registration server, a SIP proxy server,a SIP redirect server, and a SIP location server; and a storing controlunit that stores, into a memory of the communication terminal, the typeof the SIP server determined by said determination unit in associationwith the SIP server address received by said reception unit, wherein thedetermination unit determines the SIP registration server as the type ofSIP server in a case where a Register method is indicted in the responsedata, wherein the determination unit determines the SIP proxy server asthe type of SIP server in a case where a Refer method is indicated inthe response data, and wherein the determination unit determines the SIPredirect server as the type of SIP server in a case where neither theRegister method nor the Refer method is indicated in the response data.2. A communication terminal as claimed in claim 1, wherein saidreception unit uses DHCPv4 or DHCPv6 to receive the SIP server address.3. A communication terminal as claimed in claim 1, wherein saidreception unit uses a multicast packet or a broadcast packet to receivethe SIP server address.
 4. A communication terminal as claimed in claim1, wherein said transmission unit transmits an OPTIONS method forinquiring about the type of the supported SIP method which the SIPserver supports, and wherein said determination unit determines the typeof the SIP server based on the type of the supported SIP methodindicated in an Allow field of the response data to the OPTIONS method.5. A communication terminal as claimed in claim 1, wherein saidreception unit automatically receives the SIP server address when thecommunication terminal is started.
 6. A communication terminal asclaimed in claim 1, wherein said reception unit automatically receivesthe SIP server address at regular time intervals set in advance by auser.
 7. A control method for a communication terminal that carries outnetwork communication using a SIP server, comprising: a reception stepof receiving, via a network, a SIP server address of the SIP server; atransmission step of transmitting a SIP method for inquiring about atype of a supported SIP method which the SIP server supports using theSIP server address received in said reception step; a determination stepof determining a type of the SIP server based on the type of thesupported SIP method indicated in response data to the SIP methodtransmitted in said transmission step, the type of the SIP serverincluding at least one of a SIP registration server, a SIP proxy server,a SIP redirect server, and a SIP location server; and a storage step ofstoring, into a memory of the communication terminal, the type of theSIP server determined in said determination step in association with theSIP server address received in said reception step, wherein thedetermination step determines the SIP registration server as the type ofSIP server in a case where a Register method is indicted in the responsedata, wherein the determination step determines the SIP proxy server asthe type of SIP server in a case where a Refer method is indicated inthe response data, and wherein the determination step determines the SIPredirect server as the type of SIP server in a case where neither theRegister method nor the Refer method is indicated in the response data.8. A non-transitory computer-readable storage medium storing a programfor causing a computer to implement a control method for a communicationterminal that carries out network communication using a SIP server, thecontrol method comprising: a reception step of receiving, via a network,a SIP server address of the SIP server; a transmission step oftransmitting a SIP method for inquiring about a type of a supported SIPmethod which the SIP server supports using the SIP server addressreceived in said reception step; a determination step of determining atype of the SIP server based on the type of the supported SIP methodindicated in response data to the SIP method transmitted in saidtransmission step, the type of the SIP server including at least one ofa SIP registration server, a SIP proxy server, a SIP redirect server,and a SIP location server; and a storage step of storing, into a memoryof the communication terminal, the type of the SIP server determined insaid determination step in association with the SIP server addressreceived in said reception step, wherein the determination stepdetermines the SIP registration server as the type of SIP server in acase where a Register method is indicted in the response data, whereinthe determination step determines the SIP proxy server as the type ofSIP server in a case where a Refer method is indicated in the responsedata, and wherein the determination step determines the SIP redirectserver as the type of SIP server in a case where neither the Registermethod nor the Refer method is indicated in the response data.